Apparatus for testing material



Feb. 20, 1951 c. N. KIMBALL ETAL. 2,542,928

APPARATUS FOR TESTING MATERIAL Filed April 2e, 1947 s sheets-sheet 1 r rLII Feb. 20, 1951 c. N. KIMBALL. ETAL 2,542,928

APPARATUS FOR TESTING MATERIAL Filed April ze, 1947 s sheets-sheet 2491m 250 v DC 3,

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Ilan-f BY NEY.

Feb. 2o, 1951 Filed April 26, 19217` C. N. KlMBALL. El'AL APPARATUS FORTESTING MATERIAL Annn 3 Sheets-Sheet 5 INVENTORS.

Patented Feb. 20, 1951 UNITED STATES PATENT OFFICE APPARATUS FOR TESTINGMATERIAL Application April 28, 1947, Serial No. 744,120

Claims. (Cl. 17E-183) The .present invention relates in general totesting apparatus and deals more particularly with a device formeasuring the moisture content of ilour, grain and other materials ofsolid. liquid or gaseous nature.

It is well known that certain electrical characteristics of a materialvary with the relative amount of moisture contained therein, andattempts have been made in the past to measure one or more of thesecharacteristics of a given material in order to obtain some indicationof its moisture content. Apparatus has been developed, as a matter offact, which in the hands of a skilled and painstaking technician wellversed in the theory of the apparatus has been capable of yieldingsatisfactory results. Such apparatus is not suitable for general use inroutine testing, however, because of the skill and knowledge required ofthe operator and in some cases also because of the intricate characterof the test equipment itself. Very frequently in the past the moisturedetermination has been based upon comparative measurements of the testmaterial on one hand and a known standard on the other, and in all casesthe readings obtained have required mathematical conversion by means ofconversion factors, charts or curves in order iinally to obtain a figurerepresenting the percentage moisture content of the material.

It is a primary object of the present invention to provide a reliablemoisture meter which is exceedingly simple and convenient to use wherebyan unskilled, untrained operator can with a minimum of effort quicklyand accurately determine the moisture content of a given material.

Another object is to provide a moisture meter which is calibrated toread directly the percentage moisture content of the test material sothat no conversion factors or correction factors are required.

Another object of the invention is to provide a moisture meter whichregisters the correct reading on a dial or the like automatically andwithout any manual adjustment of the dial. More speciiically, it is anobject to provide a moisture meter which operates on aself-seekingbasis; and in this connection another object of the invention is toarrange for the giving of a signal responsive to a meter arriving at thecorrect reading.

A further object of the invention is to provide an improved test cellfor receiving a sample of the test material, together with an improvedarrangement for connecting the cell to the testing A 2 cells iordifferent types of materials, and facilities for altering the circuitsof the meter conveniently to suit the type of material tested.

An important feature of the invention resides in the provision of aneasily portable cup-shaped test cell for receiving a sample of thematerial to be tested. A fixed support to which the cell may easily beattached is provided, and 'whenever it is thus attached the cellautomatically is connected to the circuits of the moisture meter. Wherethe test material is of compressible character, as in the case of flourand dried milk, for example, a piston is arranged Vto compress thespecimen to a predetermined density automatically when the cell isproperly attached to the support; this contributes Very materially tothe accuracy of the results, and a safety device is provided forpreventing the taking of erroneous readings in the event the materialfor any reacircuit. Another object is to provide diiferent son is notthus compressed.

Another feature resides in the arrangements made to compensateautomatically for variation in the electrical characteristic of a sampledue to thermal changes, these arrangements being different fordifferentmaterials.

According' to the invention the test cell is incorporated in a networkcontaining a motordriven tuning device. When the network is brought to apredetermined critical condition by the tuning device, arrangements areprovided for automatically halting the device and in accordance with itsstopping point registering the percentage moisture content of thematerial in the test cell. More specifically, the network is disposed inthe plate circuit of a crystal-controlled oscillator which is adapted tocease operating when said critical condition is reached, whereupon aresponding device sensitive to the transition from the oscillating tothe nonosclllating state halts the driving motor for the tuning device.

An important feature of the invention resides in the provision made forstarting the oscillator when it isquiescent, preparatory tomaking atest.

Other objects and features will appear in the course of the followingdescription ofthe invention.

In the accompanying drawings which form a part of the specification andare to be read in conjunction therewith, and in which like referencenumerals are employed to identify like parts of the various views,

Fig. 1 is a cross sectional view of a cell for holding one type ofmaterial to be tested, showfor another type of material, showing amodied construction,

Fig. 4 is a schematic diagram of the circuit components of the moisturemeter,

Fig. 5 is a schematic illustration of the physical relationship ofvarious parts driven by the motor of Fig. 4,

Fig. 6 is a schematic diagram of a modified circuit for the moisturemeter, and

Fig. 7 is a schematic illustration of the relationship of various partsdriven by the motorof Fig. 6.

Referring iirst to Fig. l, the test cell shown therein comprises acylindrical cup or receptacle adapted to hold a representative specimenof the material whose moisture content is to be measured. It is formedof a metal tube I D one end of which is closed by a bottompiece IIscrewed into or otherwise ailixed to the tube. Within the cup therestands a hollow metal post I2. This post is disposed centrally of thetubular shell and is fastened to a disk-like insulator I3 by means ofscrews I4, the insulator in turn being clamped tightly between thebottompiece I I and an annular shoulder I5 provided on the inner wall'o'f shell I0. It will be observed that post I2 has a cone-shaped tipbut is cylindrical throughout the balance of its length.

At the bottom of the test cell there is a hollow tubular condenser I6encircling a vertical supporting stud or rod I'I. One of its terminalsis connected to the central post I2 through the medium of one of thescrews I4, while the other terminal is connected to the outer shellthrough the medium of a dished spring disk I8 positioned in a recess inthe bottom of insulator I3. The condensers outer cover of electricalinsulation and the supporting stud I1 both are made of material havingrelatively high thermal conductivity; and the condenser employs adielectric having a negative temperature coefticient (e. g., ti-y taniumoxide) the purpose of which will be made clear presently.

The upper end of the sample-receiving cup is provided with externalthreads whereby it can be screwed into an internally threaded supportingcollar I9 and thus be made to depend from said collar. The collar inturn is fastened by screws 20 to a`stationary metal support 2| which inpractice is part of the housing for the circuit components of themoisture meter. As the test cell is screwed into the collar, acylindrical ring or sleeve 22 extending downwardly from the collar isadapted to enter themouth of the cup and encircle the central post I2.'Ihis sleeve is formed of insulating material and is secured to aninwardly extending annular ange 23 which also serves as a stop for theupper end of the cup; the internal and external diameters of the sleeveare such that it slides as a piston between post I2 and the wall of thecup.

A- predeterminedv quantity of material always is employed in making atest for moisture content, this quantity being such that when it ispoured into the cup the level of the loose material falls slightly belowthe tapered portion o1' post I2. Accordingly, when the cup thereafter23, sleeve 22 compresses the sample to a predetermined density. In otherwords, so far as the density of the specimen is concerned testconditions can be duplicated repeatedly simply by employing the samequantity of material in every instance.

The central post I2 and encircling shell I0 form two electrodes betweenwhich the compressed test materal is disposed as a dielectric. Thedielectric constant of granular material such as flour, wheat orpowdered milk ranges (in the absence 0f water) from approximately 4 to6. The dielectric constant of water, on the other hand, is approximately80, and consequently it will be seen that the capacitance of the testcell if measured by a suitable instrument is a reliable indicia of therelative amount oi water contained in the specimen between theelectrodes.

To measure this factor, i. e., the capacitance of the test cell,electrical contact is made with the outer electrode through collar I9,while a terminal 26 positioned centrally of sleeve 22 and at its upperend makes contact with the inner electrode. Terminal 26 is supported bya leaf spring 21, the spring in turn being mounted on an insulator 28aflixed to the casing 2|. As the test cell is screwed into collar I9 thetip of post I2 makes the necessary electrical contact with the concaveend of the terminal and, assuming the cup is screwed tightly homeagainst ilange 23, deflects spring 21 enough to close contact 29. asshown. This contact normally is open, and should the operatorinadvertently fail to turn the sample cup up tightly, it therefore willremain open; as will be explained presently, this makes it impossible totake a reading with the meter until the situation is corrected and thusguards against inaccuracy resulting from failure to achieve thepredetermined density of the test l specimen, discussed above.

It will be convenient at this point to consider the circuits of themoisture meter more in detail, reference being had for this purpose tothe schematic diagram contained in Fig. 4. Vacuum tube 30 comprises anoscillator, the frequency of which is controlled in conventional fashionby a crystal is screwed into collar I 9 until its rim abuis flange 75 3|in its grid circuit. in the neighborhood of one megacycle is suitable.)The plate current for the oscillator tube `30 flows through a resistor32, the magnitude of this current and hence the voltage drop acrossresistor 32 being governed by the tuning of a parallel resonant circuitor network connected to the plate of the tube, which circuit is composedof inductance 33, condensers 34 to 38, inclusive, and the test cell TC.

It is well known to those versed in the art that a crystal oscillator ofthis type will operate only when the effective overall reactance of itsplate circuit at oscillation frequency is inductive; if the platecircuit is tuned toward resonance by increasing the total plate circuitcapacitance, a

point is reached at which the oscillator suddenly.

with the variable tuning condenser 31 and the (An oscillation frequencyananas trimmer condenser 38. The trimmer is employed to adjust the rangeof the meter to the testing of a particular material and once set it isnot changed. Its setting is such that even when the capacitance of thetest cell is at the highest value expected in practice (due tomeasurement of high moisture content material) the oscillator willcontinue to operate if the capacity of tuning condenser 31 is at aminimum; and the range of condenser 31 on the other hand is such that,with trimmer 38 thus adjusted, it is capable at or near its maximumcapacitance of bringing the oscillator to a halt even when thecapacitance of the test cell is at the lowest value expected inpractice.

Condenser 31 is varied by motor 42 which is adapted to turn in but onedirection, the operation of the meter being as follows. y Assuming thata measured specimen of material has been poured into the test cell andthe cell mounted as described hereinbefore, the operator thenmomentarily depresses the starting push button 43. This connects groundpotential over contact 29 to the cathode of vacuum tube 44, resulting inthe energization of relay 45 in the plate circuit of the tube 44. Uponoperation of the relay, locking contact 46 closes and by connectingground potential over contact 29 to the cathode of the tube insures thatthe tube will remain energized (and the relay hence remain operated)when the starting push button 43 now is permitted to reopen. At the sametime, the relay operates armature 41 thereby completing an obviouscircuit from the alternating current source to motor 42.

The motor accordingly begins to turn the rotor of tuning condenser 31through a gear train at a speed of approximately 4 R. P. M.

Assuming the starting position of the tuning condenser to be that shownin Fig. 5 (in which position its capacitance is at a minimum) its valuewill increase until the condenser brings the total plate capacitance ofthe oscillator 3D to the critical value at which the oscillator ceasesto operate. If the capacitance of the test cell TC is high this criticalvalue obviously will be reached with less rotation of condenser 31 thanwill be required if the capacitance of the test cell is low.

More specifically, the surge generated in the oso cillators platecircuit incident to the transition from an oscillating to anonoscillating state produces in resistor 32 a transient voltage of suchmagnitude and polarity as to bias the grid of tube 44 to cut oil. Bythus rendering the tube temporarily nonconductive, this causes relay 45to restore to normal whereupon relay contact 4E disconnects ground fromthe cathode circuit of tube 44 in order to prevent reenergization of therelay upon cessation of the transient voltage condition in resistor 32.At the same time relay armature 41 drops back, opening the motor circuitwhereupon the driving motor for the condenser halts abruptly. Thedriving motor assembly includesI a magnetic clutch which is deenergizedinstantly upon cessation of current flow, thereby improving theinstantaneous stoppage of rotation of tuning condenser 31.

The nal position of tuning condenser 31 and its associated translucentdial 48 (see Fig. 5) therefore is determined by the capacitance of thetest cell TC which, as previously suggested, is a reliable indicia ofthe moisture content of the test material forming the dielectric in thecell. In practice one section of the dial, for example the 6 half whichpasses over incandescent lamp 49, is calibrated to read directly inunits of moisture content of the flour sample.

Lamp 49 is connected to the secondary Winding of a stepdown transformer5|, the primary winding of which is controlled by relay armature 41. Itwill be seen that while relay 45 is energized and hence while the motor42 is turning. armature 41 maintains the primary winding of thetransformer open so that lamp 49 is not lighted. In other words, whilethe dialis in motion before Window 52 it is dark, i. e., the scalemarkings thereon are not visible. However, as soon as the tuningcondenser 31 reaches the point which is indicative of the moisturecontent of the sample the motor halts due to the deenergization of relay45, and armature 41 simultaneously completes an obvious circuit throughits back contact froni the alternating current source to the primarywinding of transformer 5|, causing lamp 49 to illuminate the portion ofthe dial then positioned before window 52.

The dial calibration which applies to one material may not apply toanother. Therefore, the portion of the dial passing over lamp 49 iscalibrated for one material, say flour, while the portion passing overlamp 5U is calibrated for another, said dried milk or wheat. If thesample in the test cell is composed of the second material, thetwo-position manual switch 54 will be turned manually so that itscontact 55 connects trimmer condenser 39 instead of trimmer 38 to theplate circuit of the oscillator. The function of the two trimmers is thesame, one simply being adjusted for testing one material while thesecond is adjusted for testing another. The settings of the respectivetrimmers will be substantially alike where the characteristics of thetwo materials are much alike but they obviously may differ considerably,this being especially true where the materials are so different as torequire different types of test cells, as will be explained presently.

At the same time that trimmer 39 is rendered effective due to switch 54being turned to the position applicable to the testing of dried milk,switch contact 56 connects lamp 50 to the secondary winding oftransformer 5|. Thus, when tuning condenser 31 reaches the pointindicative of the moisture content of the dried milk sampleV terials asdesired, a separate trimmer condenser being employed for each material;selection of the appropriate trimmer by a multi-position switch wouldsimultaneously connect up the lamp beneath the corresponding dial scalein the same general fashion as described above. It also is contemplatedthat a single dial scale may be employed for two or more materials incertain cases, appropriate adjustment to the use of a common scale beingmade by the trimmers employed for the respective materials.

Remembering now that the tuning condenser is brought to a halt at theproper point in its rotation due to the sudden transition of the 7oscillator from an oscillating to a nonoscillating state, it will beevident that oscillation must be reinstituted before the moisturecontent of another sample can be measured?Y In other words, assuming thesample in the test cell has been replaced by another sample, if thestart contact 43 were closed without any provision for restarting theoscillator, relay 45 would energize and, consequently, start motor 42;but the motor thereafter would rotate the tuning condenser indefinitelywithout producing any useful result inasmuch as no transitory haltingimpulse.

would be generated in the resistor 32 as hereinbefore described. True,as the tuning condenser rotated it would vary the total plate circuitcapacitance of the oscillator between certain maximum and minimum limitsbut this alone would not cause the oscillator to resume operation. v

In order to start the oscillator it is necessary to reduce the totalcapacitative reactance of the oscillator plate circuit very materiallybelow the minimum value obtainable by variation of the tuning condenser31 alone. To achieve this end, a cam B is provided on` the same shaft asthe tuning condenser, this being arranged to open contact 6I for a verybrief interval vjust as the tuning condenser is approaching its minimumvalue. The opening of contact 6l disconnects the trimmer condenser (38or 39 as the case may be) from the plate circuit of the oscillator whichmomentarily reduces the total plate circuit capacitance of theoscillator to a level which makes the inductive reactance predominate tothe degree necessary to restart the oscillator.

Cam 60 recloses contact 6I just prior to the time when the capacitanceof the tuning condenser begins to increase. This will not halt theoscillator because, as already pointed out, the combined capacitance ofthe test cell TC and the trimmer are never enough to halt the oscillatorwhen the tuning condenser is at its minimum value; however, at somepoint during the ensuing increase in the capacitance of the tuningcondenser (which point will be governed by the capacitance of the samplein the test cell) the critical value of the plate circuit capacitancewill be reached, causing the motor, the tuning condenser and the dial tohalt as explained hereinbefore.

Any tendency of relay 45 to restore to normal due to the surge generatedin resistor 32 incidental to the restarting of the oscillator is withouteffect inasmuch as a second cam 62 on the tuning condenser shaft isarranged to maintain contact 63 closed throughout the starting periodand for a brief interval afterward. In other words, even if lockingcontact 46 should be opened due to the momentary deenergization ofstate.

An alternative arrangement for restarting the oscillator is shown inFigs. 6 and 7. 'I'his employs in parallel with the tuning condenser 31 aspecial starting condenser 65, the structure of which will be apparentfrom Fig; 7. Its rotor plates comprise relatively narrow segments andits stator plates cover a plane angle in excess of by an amountcorresponding to double the width of the rotor plates. For example,rotor plates of about 30 plane angle and stator plates of 210J have beenfound satisfactory.

I'he starting condenser is so ganged with the tuning condenser that theformer reaches its maximum capacity just as the latter reaches itsminimum capacity; then throughout the ensuing 180 rotation (i. e., whilethe tuning condenser is increasing from its minimum to its maximum) thecapacity of condenser 65 remains constant at its maximum value. Afterthe tuning condenser passes its maximum and begins to decrease, thecapacitance of condenser 65 also begins to, decrease and because of thenarrow span of its rotor plates it quickly reaches its minimumcapacitance and thereafter remains at minimum level until it reaches theposition illustrated or, in other words, until the tuning condenser hassubstantially reached its minimum.

Referring to Fig. 6, it will be seen that the circuit is like the onealready described in that it has a crystal controlled oscillator 30',the output of which is impressed upon resistor 32. In the plate circuitof the oscillator in parallel with the test cell TC are condensers 31',38' and 65.

When the starting push button is depressed this energizes relay 45'which locks itself up at contact 46 while at contact 41' it starts motor42' all as discussed'hereinbefore. Trimmer 38 is set so that for everytest material falling within the expected range of moisture values thecritical point at which the oscillator ceases to operate will be reachedin the 180 rotation of the tuning condenser during which it isincreasing from its minimum capacitance to its maximum capacitance. Whenthis critical point is reached, relay 45 restores to normal, the motorhalts and lamp 48 lights as explained previously. A manual switch 54'having two (or more) positions is arranged to permit selection ofdifferent trimmer condensers whereby the meter may be adjusted quicklyto the testing of different materials, the switch also serving toconnect up the correct dial lamp in each case.

It will be understood that after a moistureregistering operation hasbeen completed the' dial remains stationary while the test material isremoved from the test cell and replaced by the next sample to be tested.When push button 43 thereafter is momentarily closed relay 45' againstarts motor 42 by closing contact 41. Tuning condenser 31 advances toand passes its maximum capacitance whereupon condenser 65 quicklyreduces the total plate circuit capacitance of the oscillator to a levelwhich will reinstate oscillation; then as it continues to rotate,condenser 65 resumes its maximum value and, as it does this, the tuningcondenser passes its minimum value and begins to increase. At some pointduring the ensuing 180 rotation of the tuning condenser (which point ofcourse is dependent upon the capacitance of the test cell TC) thecritical point is reached at which oscillation ceases, halting motor 42and causing the dial to be illuminated to indicate the new reading.

It is possible with either of the circuit arrangements described torecheck the registeredl moisture content of the sample, if desired,before the test cell is removed from its mounting, simply by reoperatingthe starter button (43 or 43'). Each time this is done the motor isstarted and the measuring cycle repeated, it being under-l stood thatthe same reading will be obtained in each case if there has been nochange in the moisture content of the sample.

'Ihe meter preferably is calibrated to read directly in terms of thepercentage moisture content of the sample at 20 C. If the temperature ofany particular sample is higher or lower than 20 C. it is well knownthat the dielectric con- Stant of that sample will be higher or lowerthan it would have been if the sample were at 20 C. It is for thisreason that the negative temperature coeilicient condenser I6 isprovidedin the test cell. The capacitance of condenser' I6 also varies withtemperature but in a reverse direction as compared to the variation inthe test specimen. If the temperature of the test specimen is above 20C. the temperature of condenser I6 will be above 20 C. by a like amount;the increase in the dielectric constant of the sample (and theconsequent increase in the capacitance of the test cell) thus will beaccompanied by a corresponding decrease in the capacitance of thecompensating' condenser I 6.

Exact cancellation of the decrease against the increase, or vice versa,requires careful selection of the compensating condenser for the testcell. It also requires efficient and rapid heat exchange between thetest material and the compensating condenser so that they always will beat the same temperature. In the case of'lour, dried milk and likematerials it has been found that the somewhat compressed condition ofthe sample in the test cell facilitates the heat exchange so that thethermal correction will be completed in all cases Within a period of oneminute after the test cell has been screwed into its supporting collar.In other words, regardless of the initial temperature of the sample ameasurement taken one minute after mounting the test cell will yield anaccurate, automatically corrected reading of the moisture content of thesample at 20 C.

The heat transfer inevitably would take place much more slowly in thecase of wheat grain and like material due to the relatively large amountof dead air space between grains. Accordingly,

for material of this kind (i. e., material having a comparatively largeparticle size) it has been found advisable to adopt a test cellconstructed along the lines of the one shown in Fig. 3. `This isgenerally similar to the cell shown in Figs. l and 2 except that thelower portion of the central post I2 is made larger in diameter wherebythe annular space between the post and the encircling cylinder I is ofsuch width that substantially all of the individual grains are directlyin physical contact with the post or the cylinder. The negativetemperature coefficient compensating condenser I6 is positioned withinthe hollow post, one terminal being connected to the post and the otherto the exterior of the shell by means of spring washer I8' as shown.

Prior to being iilled with the test material the cell naturally adjustsitself to the temperature of the ambient air, and the compensatingcondenser assumes the same temperature. A standard quantity of the grainis used for test purposes, this preferably being such that when it nowis poured into the cell its level falls below shoulder 66. If thematerial is above or below room temperature it rather quickly isadjusted to room temperature due to contact of the individual grainswith the walls of the cell; the mass of the cell is so much greater thanthe mass of test material that this results in substantially no changeof the temperature of the cell or condenser IB and, accordingly, thesample and the compensating condenser are for all practical purposes atthe same temperature. The compensating condenser functions in the Waypreviously described, whereby readings of moisture content,automatically corrected at 20 C., may be taken without any appreciabledelay even in the case of grain or other material having large particlesize.

It will be evident from the foregoing description that the dimensionsand shape of the test cell employed for each type of material may besuited to the material, all the test cells, however, beinginterchangeably attachable to the supporting collar I9 for purposes ofmaking the desired measurement. This does not mean, of course, that adifferent cell must necessarily be used in every instance, for it hasbeen found in fact that each cell usually can be employed for two ormore types of materials.

When a test cell is screwed into the supporting collar preparatory tomaking a moisture measurement it will be noted the connections are suchthat the entire lexterior of the cell is at ground potential; thiseliminates all possibility of the meter registration being affected byhand capacity effects. It also will be observed that if the test cell isnot screwed up tightly into the supporting collar, as hereinbeforeexplained, contact 29 will maintain the cathode circuit of tube 44 openand hence prevent operation of the meter until the situation iscorrected. An obvious alternative would be to place contact`29 in theplate circuit of the tube, in series with relay 45, rather than in thecathode circuit.

Although the description has dealt mainly with the use of the device tomeasure the moisture content of material. normally employed inconnection with the baking industry, it will be understood that this isintended merely to be illustrative. It may be employed equally well todetermine the moisture content of solids and liquids such as tea,coffee, tobacco, slack coal, paper, artificial silk, cotton, wool,butter, oil, etc.; moreover, by proper design of the test cell it may beused to measure the moisture in air or gases, i. e., the percenthumidity. It also will be evident that instead of being calibrated interms of moisture content the dial may be calibrated to indicatedirectly the capacitance, inductance, dielectric constant, or any otherelectrical quantity which has a counterpart capable of measurementacross the oscillator plate circuit.

Inasmuch as many possible embodiments of the invention may be madewithout departing from the scopev thereof it is to be understood thatall matter herein set forth or shown in the accompanying drawings is tobe interpreted as illustrative and not in a limiting sense.

From the foregoing it will be seen that this invention is one welladapted to attain all of the ends and objects hereinbefore set forthtogether with other advantages which are obvious and which are inherentto the apparatus.

It will be understood that certain features and subcombinations are ofutility and may be employed without reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

Having thus described our invention, we claim:

l. Apparatus for measuring the moisture content of a material,comprising a pair of spacedapart electrodes between which a sample ofthe material is disposed as a dielectric, an oscillator means controlledby s aid oscillator to operate at said critical point. and a dial forthe condenser calibrated to register directly the percentage moisturecontent of the sample at that point.

2. Apparatus as claimed in claim l, including a negative temperaturecoeilicient condenser disposed in heat exchange relation to the sampleand having its terminals connected respectively to said electrodeswhereby said condenser compensates for variations in the dielectricconstant 9i the sample due to temperature changes.

3. A device for measuring the moisture content of a sample comprising apair of spaced-apart electrodes between which the sample is disposed asa dielectric, a variable tuning condenser connected to said electrodesin bridge of said sample, a driving motor for said condenser, means forcompleting a circuit to said motor thereby to cause it to drive saidtuning condenser, means operated to halt said motor responsive to saidtuning condenser bringing the effective total capacitance of saidcondenser and electrodes to a predetermined value, and an indicatorconnected to said condenser for registering its stopping point.

4. A device as claimed in claim 3, including a lamp for illuminating theindicator, and means for lighting said lamp responsive to the effectivetotal capacitance of said condenser and electrodes reaching saidpredetermined value.

5. Apparatus for testing materials, comprising a pair of spaced-apartelectrodes between which a sample of the materialis disposed, anoscillator tube having said electrodes and sample connected in its platecircuit, inductive andcapacitative reactors also connected in the platecircuit of said tube, a motor mechanically connected to one of saidreactors for adjusting same, means for completing a circuit to saidmotor, means controlled by the motor during its initial rotation forstarting said oscillator to operate, said one reactor thereafter drivenbytsaid motor in such a direction as to tune the'plate circuit towardresonance until said oscillator ceases to operate, a device operated tointerrupt the motor circuit responsive to the cessation of saidoscillators operation, and an indicator operated by the motor inaccordance with its stopping point.

6. A device for measuring the moisture content of a sample, comprising apair of spaced-apart electrodes between which the sample is disposed, anoscillator `tube having said electrodes and sample connected in itsplate circuit, a variable tuning condenser also connected in said platecircuit, means for alternately increasing and decreasing` thecapacitance of said tuning condenser, said oscillator eilective duringsaid tuning condensers increase in capacitance to stop operatingresponsive to the capacitance of the tuning condenser reaching a valuebearing a predetermined relationship to the capacitance of saidelectrodes and sample, means controlled in accordance with thecapacitance of the tuning condenser at the time said oscillator stopsoperating for registering the moisture content of the sample, andmeanscoope'rating with said tuning condenser for restarting theoscillator in the subsequent period during which said tuning condenseris decreasing in capacitance.

7. A device for measuring the moisture content of a sample comprising apair or spaced-apart electrodes between which the sample is disposed. anoscillator tune having said electrodes and sample connected in its platecircuit, a variable tuning condenser also connected in said platecircuit, means Ior alternately increasing and decreasing the capacitanceof said timing condenser, an auxiliary condenser connected in said platecircuit, tlie capacitance or the auxiliary condenser oemg adapted toremain constant during the increase in capacitance of said tuningcondenser, said oscillator enective during said tuning condensersincrease in capacitance to stop operating responsive to the capacitanceof the tumng condenser reaching a value bearinga predeterminedrelationship to the capacitance of said electrodes and sample, meanscontrolled in accordance with the capacitance of the tuning condenser atthe time said oscillator stops operating for registering the moisturecontent of the sample, and means synchronized with said tuning condenserfor reducing the capacitance of said auxiliary condenser in the periodduring which said tuning condenser is decreasing in capacitance therebyto restart said oscillator.

8. In combination, an oscillator tube in the plate circuit of whichdevices having random values o! reactance are connectable, a variabletuning reactance connected in said plate circuit,` a driving motor forsaid tuning reactance, xiieans for completing a circuit to said motorthereby to .cause it to drive said tuning reactance, means operated bysaid oscillator for halting said motor responsive to said tuningreactance reaching the critical point at which said oscillator ceases tooperate, and a dial for registering the stopping point of said tuningreactance.

9. AIn combination, an oscillator tube in the plate circuit of whichdevices having random values of reactance are connectable, a pluralityof reactances each having a predetermined arbitrary value of reactance,means including a manually operable switch for selectively connectingany one of said last reactances to the plate circuit of said tube. avariable tuning reactance also connected to said plate circuit, adriving motor for said tuning reactance, means forcompleting a circuitto said motor thereby to cause it to drive said tuning reactance, meansoperated by said oscillator for halting the motor responsive to saidtuning reactance reaching the critical point at which said oscillatorceases to operate, and a dial for registering the stopping point of saidtuning reactance.

10. In combination, a normally conductive grid-controlled tube having aresistor in its grid circuit, an oscillator tube the output of which isimpressed upon said resistor, means for connecting random values ofreactance to the plate circuit of said oscillator, a variable tuningreactance also connected to said plate circuit, a driving motor for saidtuning reactance, means for completing a circuit for said motor therebyto cause it to drive said tuning reactance whereby said oscillator iseilective to generate a surge in said resistor responsive to said tuningreactance reaching a predetermined relationship to the ramdom value ofsaid reactance connected to said plate circuit, anda device in the platecircuit of said rst tube effective to halt saidl motor responsive tosaid surge.

. 11. A combination as in claim 10, wherein said device comprises arelay de-energized responsive to said surge. and a contact on said relayfor preventing re-energization of the relay at the 4 means for movingsaid reactor in such a way as to alternately increase and decrease itsreactance, said oscillator eective whenever said reactance is changingin a particular direction to cease operating responsive to said reactorreaching a critical point in such change, and auxiliary means associatedwith said reactor for restarting said oscillator during the change `ofits reactance in the opposite direction.

13. In combination, an oscillator tube having a variable reactorconnected to its plate circuit, means for moving said reactor in such away as to alternately increase and decrease its reactance, saidoscillator effective whenever said reactance is changed in a particulardirection to cease operating responsive to said reactor reaching a tion,and means for changing the value of said second reactor during thechange of the first reactor in the opposite direction thereby Atorestart theV oscillator.

14. In combination, an oscillator tube having inductive and capacitativereactors in its plate circuit, a motor, means for completing a circuitto said motor, means controlled by said motor during its initialrotation for temporarily adjusting the relative values of said reactorsthereby to make the over-all plate circuit reactance of said tube sopredominately inductive as to cause said oscillator to start operating,means then controlled by said motor during its ensuing rotation to tunesaid plate circuit progressively toward 14 v resonance until saidoscillator stops operating, and a device operated to interrupt saidmotor circuit responsive to the cessation of said oscillators operation.

15. In combination, an oscillator tube having inductive and capacitativereactors in its plate circuit, a motor, means for completing a circuitto said motor, means controlled by the motor during its initial rotationfor starting said oscillator, means then controlled by said motor duringits ensuing rotation for tuning said plate circuit progressively towardresonance until said oscillator stops operating, and a device operatedto interrupt said motors circuit responsive to the cessation of saidoscillators operation.

r CHARLES N. KIMBALL WILLIAM R. LEWIS. HERMAN A. STRECKER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 1,838,084 Drake Dec. 29, 19311,876,449 Cook Sept. 6, 1932 2,222,221 Burford Nov. 19, 1940 2,251,641Stein Aug. 5, 1941 2,266,114 Bartlett Dec. 16, 1941 2,271,983 La RueFeb. 3, 1942 2,297,346 Crist Sept. 29, 1942 2,304,871 f Andrews Dec. 15,1942 2,343,340 Stevens Mar. 7, 1944 2,383,480 Heyman Aug. 28, 19452,407,270 Harrison Sept. 10, 1946 2,428,700 Eilenberger Oct. 7, 19472,460,199 Taylor et al Jan. 25, 1949

